It has long been a desire to remove sulfur dioxide from exhaust gases. One widely used method of removing sulfur dioxide is limestone desulfurization. Unfortunately, limestone desulfurization processes produce massive quantities of unwanted waste water and gypsum slag whose treatment requires onerous investment.
In addition, in the limestone desulfurization process, for every 1 ton of sulfur dioxide removed, about 0.7 ton of carbon dioxide is concurrently produced.
An alternative method of desulfurization is ammonia-based. Ammonia-based desulfurization produces virtually no waste water or waste slag and produces a useful byproduct, ammonium sulfate fertilizer. Thus, it is more economic and environmentally friendly.
Chinese patents CN 1283346C and CN 1321723C disclose a process for removing SO2 from coal-fired flue gas by using ammonia as a removal agent. These patents describe reducing SO2 concentration in the exhaust to levels as low as 100mg/Nm3. The process they describe results in significant amounts of ammonia escaping from the exhaust, sometimes as high as 12 mg/Nm3.
Chinese Patent CN 100428979C discloses an ammonia-based desulfurization process using a multi-section structure, including an oxidation section, a crystallization section, a cooling absorption section, a main absorption section, and a dehydration-demisting section from bottom to top.
Chinese Patent CN 103301705B discloses an apparatus and a process for controlling fine particulate matters in desulfurized flue gas. It includes an absorption liquid demister for removing most of the droplets larger than 10 p.m, a flue gas demister for removing fine particulate matters by rewashing, and a water scrubbing process downstream from the absorption section. It describes achieving no less than 60% removal rate of fine particulate matters.
However, there is still a need for an ammonia-based desulfurization process which achieves adequate desulfurization of exhaust gases while simultaneously inhibiting ammonia escape and aerosol formation.